Based on my experience so far mine has gone off with about 2.5 gallons left. I don't have the DTE thingy on mine, so I'm just going by how much it takes to fill back up when I make it to the station. I'll be filling up at lunch because the light came on this morning, so I'll let everyone know if anything changes.
ETA: It took just barely over 17 gallons to fill this time.